The present invention relates to corrosion protection by paint. More particularly, the invention provides a paint containing a multitude of extremely thin platelets having a large area/thickness ratio, which serve to greatly reduce permeability of said paint when applied and dried.
Paints are decorative and protective coatings made of finely ground solids, including binders, pigments and other additives such as dryers, plasticizers, emulsifiers, anti-foamers, leveling agents and thickeners suspended in a liquid carrier known as a filmogen. The liquid component may be a resin dissolved in a volatile thinner or organic solvents such as drying vegetable oils or solvents respectively, or the carrier may be an aqueous solution. Oil based paints have had difficulties in meeting certain new clean-air laws which mandate that the amount of volatile organic compounds in paint be lowered. Consequently, manufacturers have concentrated much effort on effecting improvements in water-based formulations.
Besides standard paints, special paints are available for particular requirements, for example marine use, chemical resistance, low-odor paints, fire retardation, traffic, and heat reflection.
Although paint has many uses, the application of prime economic importance is the protection of steel surfaces against corrosion, which causes billions of dollars damage yearly.
While other processes such as hot-dip galvanizing provide better protection for steel, paint has many advantages, one of which is that it can be applied in many situations, capital investment required is negligible, the item to be protected may be large or small, fixed or mobile, and paint can be applied to structures where processes such as hot dipping cannot be used.
It is general considered to be good practice that in order to obtain strong adhesion of a paint to a substrate, the latter is modified before painting either by mechanical means such as roughening, or by chemical means such as the application primers or etching, the aim being the development of a durable interface between the paint and the article being coated.
The process of corrosion of painted metals is initiated by permeation of humidity, corrosive salts and/or solvents into the paint layer, causing softening and degradation of the paint. Subsequently the attacking substance reaches the paint/metal interface, which fails thereafter. This allows the invading substance to attack the metal itself. Clearly it would be advantageous if possible to halt such penetration in the outer or middle portion of the paint layer before the paint/metal interface is endangered.
Nanoplatelets have been used in the plastics industry for purposes of mechanical reinforcement, fire retardancy and in reducing permeability. Alumina-silicate nanoplatelets are presently available commercially, at the moderate cost of about $5/kg. Published prior art includes use of platelets in pigments but for purposes other than that proposed in the present invention.
Andes in U.S. Pat. Nos. 5,858,078 and 5,985,020 discloses a titanium dioxide pigment based on platelets made thereof. Schmid, in U.S. Pat. Nos. 6,139,614 and 6,193,794 has recently proposed the use of coated iron oxide or silicatic platelets for producing goniochromatic luster pigments. Such applications are intended to produce special optical effects but are not intended to reduce the permeability of a paint coating.